A ripple-control DC-DC conversion is a method in which a voltage ripple at a power stage is inputted to a comparator and a comparator output signal is used as a switch driving signal, and the switching is a self-excited operation. Thus, since it is not require a clock supplied thereto from the outside and hence the operating speed is not limited by a clock frequency, the conversion has a feature of rapid response to load variation. Besides, the conversion has an advantage of reducing the circuit size because it does not require using a pulse width modulation (PWM) modulator or a compensator.
An example of a conventional ripple-control DC-DC converting circuit is described below. A high-side switch and a low-side switch, which complementarily operate, step down an input voltage applied to an input terminal to a voltage lower than the input voltage. The stepped-down voltage is smoothed by an inductor and a capacitor and is outputted from an output terminal.
When an output voltage is directly fed back, a loop operation is unstable due to a phase rotation of 180° by the inductor and the capacitor. In order to solve the problem, in the conventional ripple-control DC-DC converting circuit, a ripple voltage is generated by passing a difference voltage between the stepped-down voltage and the output voltage through a low-pass filter composed of a resistor Rf and another capacitor Cf. The ripple voltage is fed back and is compared with a reference voltage in a comparator. An output voltage of the comparator is used as a driving signal for the high-side switch, and a signal obtained by inverting a comparator output with an inverter is used as a driving signal for the low-side switch.
When the time constant Rf*Cf of the low-pass filter is set sufficiently large, the waveform of the ripple voltage is similar to the waveform of an inductor current, so that the phase rotation in the open loop amounts to 90°. As a result, the feedback loop operates stably.
As mentioned above, the conventional ripple-control DC-DC converter has an advantage that the stable operation can be achieved by a simple circuit but has a disadvantage that the time constant of the low-pass filter needs to be set large.
For example, in a ripple-control DC-DC converter that operates with a relatively high switching frequency of 4 MHz, an external resistor and a capacitor constituting the low-pass filter have large values of Rf=100 kΩ and Cf=150 pF. In order to apply the conventional technique to a DC-DC converter having a switching frequency lower than 4 MHz, elements having values larger than above are required, and it is very difficult to incorporate the low-pass filter into a DC-DC converter IC.